The present invention relates generally to wireless local area networks, and more particularly to control of quality of service in overlapping wireless local area network basic service sets.
Wireless local area networks (WLANs) provide advantages both for network provisioning and for customer services. For a network provider, a WLAN reduces required runs of infrastructure cabling. For a network user, a WLAN provides ready access for mobile devices such as laptop computers, personal digital assistants, and cell phones equipped with data capabilities. WLANs may be configured via various network schemes. Some are proprietary, and some follow industry standards. At present, many widely-deployed WLANs follow the IEEE 802.11 standards.
WLANs may be further classified by architecture. In a mobile ad-hoc network, wireless devices, such as laptops outfitted with wireless modems, communicate directly with each other in a peer-to-peer mode over a radiofrequency (RF) channel. In an infrastructure network, wireless devices communicate with an access point via a RF channel. A home WLAN typically will be served by a single access point, such as a wireless router. The wireless router then connects to a packet data network via a broadband access service, such as digital subscriber line, cable, or fiberoptics. Wireless devices may also communicate with each other via the single access point.
To provide RF coverage over a wider area, such as in an airport or commercial complex, multiple access points are often deployed. The RF coverage areas of multiple access points may overlap. In some instances, the overlap is intentional to provide seamless coverage. In other instances, the overlap is unintentional since the boundaries of RF coverage areas are not sharply defined. RF coverage areas may also overlap if more than one network is operating in the same location. For example, in a commercial environment, competing network providers may be offering services in the same, or an adjacent, location. In a residential environment, such as an apartment complex or a neighborhood in which houses are close together, WLANs independently operated by neighbors may overlap in RF coverage area.
In a widely-deployed WLAN protocol, devices communicate with each other over a common channel on a contention basis. That is, each device attempts to independently acquire the channel. At a given instance, if there are multiple contending devices, the device which actually acquires the channel is governed by a carrier sense multiple access/collision avoidance (CSMA/CA) protocol and a random backoff mechanism. When a device wants to transmit, it first senses the medium (RF channel) to determine whether the medium is busy. The medium is busy if there is already data traffic on the RF channel. If the medium is determined to be busy, the device defers until the end of the current transmission. After the device senses an idle channel, or prior to attempting to transmit again immediately after a successful transmission, the device defers transmission for an additional delay period. If the channel remains idle throughout the delay period, the device then selects a random backoff interval. The random backoff interval comprises a number of time slots, and a timer counts down the number of time slots. If the channel remains idle throughout the random backoff interval, the device acquires the channel at the end of the random backoff interval and starts to transmit.
For packet data transmission, the CSMA/CA mechanism provides efficient channel utilization when the channel is shared by multiple devices, and the traffic load is moderate. WLANs, however, are increasingly being used to transport multimedia services (voice, video, and data). Voice and video transport have more stringent network transport requirements (quality of service) than data. Quality of service (QoS) parameters include packet loss, delay time, and jitter. The basic CSMA/CA mechanism does not distinguish priorities among the different classes of multimedia services. Since they all contend for the channel on an equal basis, video and voice quality may be degraded when the channel is heavily loaded.
Enhancements to the basic CSMA/CA provide various degrees of QoS support. In the scheduled access mode, devices are individually polled by a channel access coordinator. For example, in a WLAN controlled by a single access point, the access point may serve as a channel access coordinator to coordinate the devices in the network. Under scheduled access, devices do not need to execute the carrier sensing and backoff mechanisms. At a given instance, only the one device being polled is allowed to transmit. Due to the deterministic nature of scheduled access, QoS, in some instances, may be guaranteed by the polling schedule. Scheduled access, however, is complex to manage in deployments and has not been widely deployed.
An alternative mechanism for QoS support is referred to as the enhanced distributed channel access (EDCA) mechanism. EDCA recognizes four different traffic service classes (background, best effort, video, and voice). Priority is provided via a set of configurable parameters (EDCA parameter set) which are transmitted by an access point to the mobile devices under its control. EDCA, however, may not be efficient or effective if two or more access points have overlapping RF coverage areas. For example, if there are two access points, a first set of mobile devices is controlled by the first access point, and a second set of mobile devices is controlled by the second access point. The first access point controls (to a degree) QoS among the traffic streams transmitted by the set of mobiles under its control. Similarly, the second access point controls (to a degree) QoS among the traffic streams transmitted by the set of mobiles under its control.
If the two access points operate independently, however, traffic streams from mobiles in the first set and traffic streams from mobiles in the second set may compete equally for channel access if the mobiles are located in the region of overlapping RF coverage. For example, a high-priority video stream transmitted by a mobile controlled by the first access point may compete equally for the channel with a high-priority video stream transmitted by a mobile controlled by the second access point. In some instances, a high-priority traffic stream transmitted by a mobile controlled by the first access point may compete equally for the channel with a lower priority traffic stream transmitted by a mobile controlled by the second access point.
Since there may be more devices competing for channel access in the overlapping RF coverage area (when compared to the number of devices within an equivalent area covered by only one access point), channel access deferral due to back-off and collisions are more likely to occur; hence the overall channel access efficiency may be reduced. Traffic streams from mobiles in the first set and traffic streams from mobiles in the second set may also compete unfairly for channel access if the mobiles are located in the overlapping RF coverage area. If the channel access parameters of the first set are configured differently from those of the second set such that traffic streams in the first set have a higher probability of winning the channel access than traffic streams of the same priority class in the second set, traffic streams in the first set have an unfair advantage. In some instances, the channel access parameters may even be configured such that a low priority traffic stream in the first set may have higher channel access probability than a high priority traffic stream in the second set. What are needed are method and apparatus for controlling QoS for wireless devices in regions of overlapping RF coverage.